1. Field of the Invention
This invention relates generally to holography and more particularly has reference to improving holograms by eliminating or minimizing unwanted flare effects.
2. Description of the Prior Art
In modern aircraft, both military and commercial, it is important that certain information be clearly and unobtrusively presented to the pilot while he is viewing the outside world. The head-up display (HUD) has been developed for this purpose and provides relevant scale, alphanumerics, symbology, gun sight reticle (in fighter aircraft) and other information displays superimposed on the pilot's forward field of view. With conventional HUD designs, the information display is generated on a high-brightness cathode ray tube and projected through a relay lens system to a transparent combiner screen, typically a partially silvered mirror, which is located between the pilot and the aircraft windscreen. The combiner reflects the projected images to the pilot's eyes while affording him an unobstructed view of the outside world through the combiner and windscreen.
One combiner recently developed for head-up displays includes a holographic film for reflection of the projected image. This film comprises one or more layers of photoreactive gelatin which have been exposed by a holographic process to record a holographic fringe pattern therein. A holographic film has better reflection and see-through capability than a partially silvered mirror.
In making reflection holograms, a thin layer of photoreactive gelatin on a glass substrate is exposed to in-phase laser beams of opposite direction which intersect at the gelatin layer. The intersecting beams set up an interference pattern which is recorded in the gelatin as a sinusoidal modulation of the index of refraction of the gelatin. A pattern of holographic fringes corresponding to the modulation of the index of refraction defines the hologram. The fringes are parallel to the line bisecting the angle between the two laser beams. Where the bisector is parallel to the surface of the gelatin, the fringes are parallel to the surface. Where the bisector is at an acute angle to the surface, the fringes are slanted.
Transmission holograms are produced by laser beams which are directed at the gelatin from the same side. If the bisector of the beams is perpendicular to the gelatin surface, the fringes are also perpendicular to the surface.
It is usually desirable to construct a hologram with zero degree fringes, i.e., fringes that are parallel to the surface of the gelatin. However, design constraints occasionally prevent that arrangement. In those cases, the fringes intersect the surface and form a slant fringe pattern. One problem with slant fringe holograms is that they produce extraneous diffraction images which are analogous to the diffraction produced by a transmission hologram. We call the extraneous diffraction "flare". The extraneous diffraction is caused by surface effects. The desired hologram formed by the volume holographic fringes is aligned at an angle and intersects the surface of the holographic material to form a thin phase grating therealong. This surface effect is completely different from surface deformation effects which can be eliminated by index matching and is also completely different from surface reflection flare which can be removed by selectively moving components of the hologram construction system during recording (See U.S. Pat. Nos. 4,456,328; 4,458,977 and 4,458,978). Procedures described in the literature for making a holographic exposure at a shortened wavelength, e.g., 4880 .ANG. v. 5145 .ANG., had an incidental effect on slant fringe flare but were never recognized as acheiving that result. Zero degree holograms are not subject to the flare effects because the fringes do not intersect the surface of the gelatin.
One result of flare is the diffraction of light spots, such as the beams from airport runway lights, which are viewed through a holographic combiner. Diffraction produces multiple order images of the light sources. Under certain conditions, particularly in night landings, substantial portions of the pilot's field of view may be obscured by the flare from runway lights.
Another use of holographic films has been in visors designed to protect eyes from damaging light exposure, such as might be encountered from an incoming laser beam. A holographic element incorporated into the visor diffracts and red-directs incoming laser light out of the field of vision. By suitable control of the light used to generate the hologram (angles, wavelengths, etc.), the degree of reflectivity/transmissivity of the hologram can be selectively determined and a cone of total reflection for a given wavelength can be provided to protect the user's eyes from incident damaging light beams. It is essential that the hologram be nearly 100% efficient in diffracting the damaging wavelengths of light at an angle so that they do not reach the eye. Otherwise, with an extremely intense beam, such as a laser beam, light which is not properly diffracted may be enough to damage the user's eyes. Improper diffraction can be caused by the flare effect.
As stated above, flare in slant fringe holograms is caused by surface effects. The periodic variations in the index of refraction at the surface of the gelatin layer cause the surface to act as a planar phase grating. The grating breaks up an incident light beam into two beams which are diffracted at equal but opposite angles to the incident beam. These beams differ from the reflection beam produced by the volume hologram. Hence, single points of light viewed through the hologram produce two first order diffraction beams even though the primary beam is reflected. In a laser eye protection device, the hologram protects against laser light directed at the user's eyes. A laser beam hitting the hologram from some other direction may be directed toward the eyes as a result of the flare effect, i.e., one of the flare points may fall in the cone of protection, thus exposing the user's eyes to possible damage. The present invention is directed to eliminating these unwanted flare points.